The Metanefridium Are a type of excretory gland that is found exclusively in invertebrates belonging to the group of annelids or worms, some mollusks and atroopods.
Metanefrides have the function of eliminating waste resulting from the metabolic process and are in charge of keeping regulated the concentration of all kinds of substances inside the body of the worms (Britannica, 2017).
Within the animal kingdom both vertebrates and invertebrates can be found. This second group is characterized by the fact that the animals that compose it do not have a spine or an articulated internal skeleton. It includes worms or annelids, mollusks and arthropods.
Most invertebrates have a waste excretion system composed of nephri- dians, which can be protonefridios (flam- ing cells) or metanefridios.
These systems are glands that perform a function similar to that of the kidneys in other animals (Hine, 2015). Unlike the protonefridios, the metanefridium tubules lack flamboyant cells and open directly into the body cavity known as coelom.
The cilia that line the inside of each tubule create an empty space through which fluids can be drawn to the outside.
During this process, the cells that line the walls of the metanefridium are responsible for reabsorbing important nutrients still contained in the fluids as they pass through the tubule.
Structure of metanefridia
Metanefridios are glands that fulfill the function of the kidneys in worms or annelids. Its structure is composed of a group of tubules, usually a pair of them for each coelom (cavity dedicated to facilitate the transport of substances inside the body of the annelid). These tubules are open at both ends.
The first end of the tubules is located inside the cavity of the coelom, forming a structure similar to that of a funnel.
This end is known as nephrostoma and has several cilia around it, directing the flow of substances into the coelome. The other end is called nefidiopore and is located on the outside of the body.
The nephrostoma is a duct full of cilia that opens inside the coelom. On the other hand, the nephriodoporo counts on numerous glands, of this form its size can increase or diminish thanks to the action of vesicles that help to eliminate everything that is in its interior.
The tubules of the metanefridios have the capacity to transport liquids by means of a system of pumping and the action of the cilia that are in them.
By being able to transport water, they allow the excess of ions, toxins, wastes and hormones to be eliminated through nephriodopore (Schmidt-Nielsen, 2002).
The urine produced by the process of filtration of the blood of the annelids is transformed into secondary urine with the help of the cells that cover the metanefridium.
In this way, the chemical composition is regulated inside the body of the annelids, extracting only the products that do not contribute anything and whose concentration is high.
Function
The metanefridios fulfill the same function as the flaming cells in other invertebrates. These are responsible for removing debris from the body of annelids, some mollusks and arthropods (Fanjul & Hiriart, 1998).
They are excretory structures more complex than the protonefridios, since they are open at both ends, giving faster and easier passage to the fluids that are in them. On the other hand, they are highly vascularized, so they can contribute to the urine manufacturing process.
By having a double opening that allows them to be connected to the outside and to the coelom, the metanefridios are able to receive the waste materials collected in the coelom, transporting it, filtering them a second time and finally sending them to the outside for its elimination. In other words, the metanefridians drain the liquid present inside the coelom.
When the waste fluids or urine pass from the coelom to the metanefridium, its concentration is isotonic, however, when passing through the tubules of metanefridium, all salts are removed, leaving the urine as a more diluted substance.
In this way, the function of metanefridios can be understood as if they were kidneys, since they aim to filter and reabsorb the substances contained in the urine, forming an initial solution by means of a filtration process that later will be modified by A process of reabsorption as it passes through the tubules (Spain, sf).
Appearance
The appearance, shape and size of both coelom and metanefridia may vary depending on the species of invertebrate that has them.
Annelids or worms
In the annelids the coelom is elongated, therefore, its body counts on different sets of metanefridios, usually a pair for each ring of the body.
Molluscs
In the case of molluscs, the coelom appears as a cavity that includes both the pericardium and the kidneys, therefore, the metanefridios are seen as if they were a kidney in the body of the molluscs.
Arthropods
Few arthropods use the coelom system and metanefridium to carry out the excretion process of waste substances.
However, those who do have small nephrid tubes connected to the coelom, which is a sac of small size and thin walls, connected to the inner terminal of the excretory tubules or metanefridium.
Regardless of the appearance or size of the coelom and metanefridiums, the functions carried out by this system are always the same within the body of any invertebrate.
This is how the metanefridians are responsible for evacuating the solutions contained inside the coelom, slowly moving them outwards, while filtering the nutrients still present.
In this way, metanefridia will always be linked to the process of urine formation, its filtration, reabsorption and subsequent evacuation to the exterior of the body (Recio, 2015).
References
- Britannica, T.E. (2017). Encyclopædia Britannica . Retrieved from Nephridium: britannica.com
- Spain, G. d. (S.f.). Biosphere Project . From EXCRETORES INVERTEBRADOS SYSTEMS: recursos.cnice.mec.es
- Fanjul, M. L., & Hiriart, M. (1998). Nephridia. In M. L. Fanjul, & M. Hiriart, Functional biology of animals (Pages 507-509). Century Vientiuno Publishers.
- Hine, R. (2015). Metanephridium. In R. Hine, A Dictionary of Biology (Page 368). Oxford: Oxford University Press.
- Price, C. G. (November 26, 2015). Paradais-Sphynx . Obtained from Excretory apparatus of animals. System types and examples: paradais-sphynx.com
- Schmidt-Nielsen, K. (2002). Animal Physiology: Adaptation and Environment. New York: Cambridge University Press.